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Impact energy dependence of Al13 cluster deposition on Ni(0 0 1) surface
Surface Science 512 (2002) 128–134 www.elsevier.com/locate/susc
Impact energy dependence of Al13 cluster deposition on Ni(0 0 1) surface Y.X. Wang
b
a,b
, Z.Y. Pan a,b,*, Q. Wei a,b, A.J. Du Y. Xu a,b, Y.K. Ho a,b
a,b
, Z. Huang
a,b
,
a Institute of Modern Physics, Fudan University, Shanghai 200433, China Ion beam Laboratory, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
Received 19 June 2001; accepted for publication 20 March 2002
Abstract In this paper, the influence of the impact energy on the initial fabrication of thin films formed by low energy cluster deposition was investigated by molecular dynamics simulation of Al13 clusters depositing on Ni(0 0 1) substrate. In the case of soft-landing, (0.01 eV/atom), clusters are rearranged from Ih symmetry into fcc-like clusters on the surface. Then they aggregate each other, which result in thin film growing in 3D island mode. While, growth will be in layer-by-layer mode at the impact energy of a few electron volt due to the transient lateral spread of cluster atoms induced by dense collision cascade. This effect has been traced to collision cascade inside the cluster, which is enhanced by collision with a hard Ni substrate. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Clusters; Growth; Epitaxy; Aluminum; Nickel
1. Introduction The investigation of the low-energy cluster beam deposition (LECBD) is of great importance in synthesizing nanostructured thin films of novel properties and has attracted considerable attention [1–3]. The growth of a film was known to be a complicated process, which relates to its structure and properties. In general, there are three growth modes for metal particle deposition on metal substrate, that is, the Frank–van deer Mere mode * Corresponding author. Address: Institute of Modern Physics, Fudan University, Shanghai 200433, China. Fax: +86-21-6510-4949/6564-3815. E-mail address: [email protected] (Z.Y. Pan).
(FM, layer-by-layer growth), the Volume-Weber mode (VW, three-dimensional islands growth), and the Stranski–Krastanov (SK, three-dimensional island growth after layer-by-layer growth). Most of the earlier studies were concentrated on the equilibrium thermodynamics and kinetics of growth [4,5]. With these theories, it is difficult to predict the growth mode and dynamics of LECBD, because non-equilibrium dominate the growth process and many parameters, e.g. the specific cluster and substrate material, the impact energy and the temperature, etc., affect the growth mode. In consequence, a diverse behavior of thin film growth can be observed with the change of those parameters mentioned above. So it stimulated a number of theoretical and experimental
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 6 9 1 - 6
Y.X. Wang et al. / Surface Science 512 (2002) 128–134
studies [1–3,6–8]. Heiz group investigated the sizedependent catalytic properties of metal cluster (Ag, Cu and Ni) deposition on the MgO surface [9]. The group of Palmer impacted size-selected cluster ions ðAg n Þ on a covalently bonded substrate (graphite) over the energy range 15–1500 eV. The penetration of cluster into surface versus diffusion and aggregation are studied. It highlights the need to consider the collision geometry associated with cluster–surface interaction [10–12]. However, the mechanisms of thin film growth, especially, the importance of interactions of cluster–cluster and cluster–substrate, which is strongly dependent on the growth conditions, are still required to clarify in detail [5,7,8]. In the present work, Al13 clusters of Ih symmetry were chosen to deposit randomly on the Ni substrate by varying the impact energy, Ein , from 0.01 eV/atom (softlanding) to 10 eV/atom (low energy). In this case, the cohesive energy of Al cluster (2.791 eV/atom) is much lower than that of the substrate (4.435 eV/ atom). The influence of incident energy on growth mode of thin films was investigated by molecular dynamics simulation (MDS). Special attention was put on studying the effect of collision dynamics induced by cluster impact on the growth mode of cluster-assembled thin films.
2. Computational model In the molecular dynamics program, classical equations of motion were set at atomic level and integrated by using the leapfrog form of the Verlet algorithm[15]. The interactions between Al–Al, Al–Ni and Ni–Ni atoms were modeled by Finnis– Sinclair potentials of Ackland et al. [13] which was modified further at close atomic separations by Gao [14]. The Al13 cluster of Ih symmetry was selected as the projectile, which was initially located at a sufficient distance above the Ni surface where the interaction between the cluster and the substrate was negligible. Then it impacted normally onto the surface. The Ni(0 0 1) substrate consisted of 12 atom layers with periodic boundary conditions at the edges (x–y plane). Each layer contained 128 atoms (8 8 unit cell). In order to study the dependence of impact energy on the growth dy-
129
namics, the temperature of the substrate was set at 0 K. To explore the atomic-scale growth mechanism of Al cluster depositions, a simulation model was designed as following. Firstly, a single cluster impingement has been studied in Section 3.1, which was expected to elucidate the dynamics of cluster deposition as well as the lateral migration of supported cluster atoms at the earlier stage. Then, thin film growth of multi-cluster deposition was carried out in Section 3.2, where the Al13 cluster was arranged to vertically drop on the Ni surface one after another. Their impact position and the orientation were chosen randomly.
3. Results and discussion 3.1. Single Al13 cluster interaction with Ni surface In our previous work [16], it was found that Ni substrate is hardly damaged for Al13 cluster impingement with energy lower than 10 eV/atom. Thus, deposition energy of 6 eV/atom was chosen to study the deposition dynamics of Al clusters impacting on Ni(0 0 1) surface. The top and side views of atomic locations of the simulation system at various instants of time during a collision event are shown in Fig. 1. As the Al13 cluster reached the Ni surface at about 0.1 ps, it was deformed severely. At t 0:2 ps, it impinged into fragments mainly being single atoms. Then, these atoms widely outspread as single atoms during the time interval (0.2–0.8 ps). At the end, they resided on the surface as an epilayer. To describe the lateral spread of cluster atoms quantitatively, the radius of gyration ðrg Þ of the cluster atoms [8] was calculated as a function of time, as shown in Fig. 2. It was defined as follows: 1
rg ¼ ðhri2 iÞ2 Here, ri is the transverse distance between the ith cluster atom and the center of mass of the cluster (MCC). It can be seen from Fig. 2 that at t 0:8 ps, rg got to the equilibrium value about 2.93 aNi 0 where aNi 0 is the lattice constant of Ni metal. It is much more than rg ¼ 0:73 aNi 0 at t ¼ 0, i.e. the radius of the original cluster. While, lateral
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Fig. 1. Snapshots of an Al13 cluster of In symmetry impacting on Ni(0 0 1) surface. The incident energy is 6.0 eV/atom. At each instant, the left shows the side view and the right for the top view. The black dots represent Ni atoms, and open circles for Al atoms.
Fig. 2. The radii of gyration of the cluster atoms versus time for an Al13 cluster deposition on (0 0 1) Ni surface with energy of 6.0 eV/atom.
movement of the MCC was only 0.17 aNi 0 . Furthermore, the distribution of transverse spread of the cluster atoms ðri Þ, at t ¼ 0:89 ps is exhibited in Fig. 3, where the statistics were accumulated over eight events and twenty time steps for each event. At the moment, most of cluster atoms have moved more than half of a lattice constant. Some of them
Fig. 3. The distribution of transverse spread of the cluster atoms. The solid line is the distribution at t ¼ 0:89 ps. And the dashed is that of a free Al13 cluster.
were located far from other cluster atoms and the separation between them is beyond the cut off . Among them, the distance of the potential, 4.96 A . We return to Fig. farthest spread ri is up to 16.0 A 2. It can be inferred from the slope of the curve that cluster atoms obtained the maximal transverse velocities at about 0.15 ps. The simulation
Y.X. Wang et al. / Surface Science 512 (2002) 128–134
also showed that within this short time interval, the acceleration process was almost completed and the transverse kinetic energies of cluster atoms relative to the MCC, Epi , raised to the maximum. While, the vertical translational kinetic energy of cluster atoms dropped into the minimum, being close to zero. For those cluster atoms moving far from MCC, the values of Epi were above 0.5 Ein , among them, the highest was close to 0.96 Ein . It illustrates that cluster atoms obtained enough lateral kinetic energy relative to the MCC, and then, spread widely through transient diffusion due to ballistic collision. An epilayer was thus formed. Then we varied the orientation and the impact position of the incident cluster relative to the surface. It was found that the deposition morphology was almost unaffected by the collision geometry, which is different from that on a covalently bonded graphite [11]. Furthermore, the factors, resulting in spreading of cluster atoms so much, were investigated. The local heating caused by cluster impact was first examined. The study of energy transfer showed that the internal kinetic energy of the substrate was increased to 40 eV after deposition [16]. Assuming it was close to the equilibrium state, the corresponding kinetic temperature was around 100 K, since the impinging energy was lower (6 eV/atom). Besides, the time, at which cluster atoms obtained the maximum transverse velocity, was about 0.15 ps. It is much short than the time scale of thermal migration. Therefore, we conclude the impact induced collision cascade takes a leading role, which was related to both cluster–cluster and cluster– surface interaction. Then, we performed two new simulations, which were similar to the previous one to investigate the two effects in detail, respectively. In one simulation, the interaction among the cluster atoms was neglected. The calculation showed the rg was just 1.50 aNi 0 at the end of the event. For the other simulation, the interactions between cluster atoms have been considered, but Al replaced the Ni substrate. In this case, rg was still small, which was calculated to be 1.36 aAl 0 , where aAl is the lattice constant of Al metal. It can 0 be understood qualitatively as follows. The Ni substrate is much harder than Al [8]. Not the same as Al, the Ni substrate does not undergo sub-
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Fig. 4. Collision between two cluster atoms leading to the lateral acceleration of cluster atoms. Front atom A in the Al cluster is assumed to be reflected from the hard Ni surface having the maximum velocity V0 , where V0 is the initial velocity of projectiles. In the left sketch, the collision is inspected from a reference frame moving with atom B. The reflected atom A having the velocity 2V0 collided with atom B. After the collision, both atoms obtained the velocity component parallel to the surface. The right sketch indicates the transformation back to the laboratory frame. The cluster atom gained the maximum lateral velocity, V0 , if it exits at the angle of 45° with the incident direction in the moving frame.
stantial yielding under the Al cluster impact, which makes the substrate response for more elastic stiffness and promotes the cluster atom migration. Furthermore, a rough estimation was performed from the classical two-body collision shown in Fig. 4. Assuming that an Al cluster atom (atom A) reflected normally from the hard substrate with the maximum velocity V0 , where V0 is the initial translational velocity of the projectile, it may collide with another later arriving Al projectile (atom B). Under the condition of one head-on collision between the two moving Al atoms, both can be accelerated in the direction parallel to the surface. This process can be illustrated in a reference frame moving with the cluster velocity. Atom A moving with velocity 2V0 collides with atom B, the two Al atoms must move at right angles to each other after the collision. If one atom exits near the direction of p=4 with the initial beam direction, it will obtain the maximum lateral velocity, V0 , in the laboratory frame, which is close to our simulation result. On the other hand, in the case of monatomic impact, the projectile atom cannot get
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lateral momentum when it collides with a hard substrate. Whereas, the moving Al atom can also obtain the lateral pffiffiffivelocity which maximum value is less than V0 = 3 if it collides with a static Ni atom. Therefore, it is believed the collisions between moving cluster atoms influenced by the hard Ni substrate are important factors in contributing to cluster atoms obtaining high transverse velocity. Consequently, these atoms can widely spread over the Ni surface in the form of single atom motion. Compared with large clusters [8], it seems that the interactions are more efficient for small clusters to spread. As a comparison, a cluster with energy of 0.01 eV/atom (soft-landing) was deposited on the same substrate. It was found that collision cascade did not take place. The Al cluster was supported on the Ni surface as a bulk molecule and rearranged into fcc-like structure due to strong Al–Ni interaction [16]. The transfer between the potential energy and the kinetic energy of this system was completed within 2.0 ps. rg , corresponding to this fcc-like cluster, was calculated to be 0.84 aNi 0 , close to that of a free Al13 cluster, 0.73 aNi The energy 0 released upon formation of the cluster–surface bond is much smaller than the cohesive energy of the Al13 cluster. And then, cluster atoms could not get over the binding among them to spread. 3.2. Simulation of initial fabrication of Al thin film growth We know from Section 3.1 that there is a moderate energy range in which collision cascade results in wild spread for cluster atoms over the surface. Is the behavior of collision dynamics able to influence thin film growth in the moderate energy range? Since the limitation of computational capability, only 22 clusters were calculated to deposit on the surface one after another at the energy of 6 eV/atom. It is known that the mainly dynamics process of deposition has been completed within 3 ps. In addition, the transient process plays a leading role in the metal deposition [17]. So, in our simulation, the deposition was arranged as follows to mimic the experimental conditions. After one Al13 cluster impacted on the surface, the trajectory integration lasted 10 ps to
Fig. 5. The structure feature of Al13 cluster impacting on Ni(0 0 1) surface with energy of 6 eV/atom. (a) The snapshot after 8 clusters impacted on the surface. (b) The snapshot after 22 clusters impacted on the surface.
reach a quasi-equilibrium state. After that, the next cluster began to drop. Fig. 5a is the snapshot after 8 clusters deposited on the Ni surface. It can be seen that all the clusters break away. The atoms in different clusters were intermixed each other. Most of them resided on the surface forming an Al epilayer. For this epilayer, the coverage of Al atoms ðhÞ was 69%. After 22 clusters, in turn, impacted on the surface, two Al epitaxial layers were constructed as shown in Fig. 5b. To monitor its growth mode, we calculated the diffraction intensity [18] as a function of the coverage of cluster atoms, and plotted it in Fig. 6. In Fig. 6, there is a
Fig. 6. Diffraction intensity versus the coverage of cluster atoms for Al13 clusters deposition on Ni(0 0 1) surface with energy of 6 eV/atom.
Y.X. Wang et al. / Surface Science 512 (2002) 128–134
peak near h 1:0. This indicates the initial thin film growth is layer-by-layer like during Al cluster deposition. It can be understood from Section 3.1. The cluster was breaking into pieces and the projectile atoms obtained transverse momentum due to the impingement. Then the cluster atoms experienced transient diffusion to move far from the impact point over the surface. The supported clusters were coalesced each other. Hence, the layerby-layer growth of deposited clusters was thus formed. This mechanism of transient mobility for film growth at low temperature is different from that for film growth in atomic deposition [19,20]. In the case of atom deposition, the transient diffusion stems from the need to dissipate the kinetic energy of single atom and the potential energy released upon formation of the atom–surface bond. It might be interesting to study how does the growth mode of cluster-assembled films depend on the impact energy? Therefore, the simulation was repeated again, but the deposition energy per atom was 0.01 eV. As expected, no fragments generated. Each cluster just rearranged into fcc structure. Additionally, we did not find the collective motions of deposition clusters. The neighbor clusters simply aggregate each other into big fcc island [7]. Subsequently, fcc islands grew up through the aggregation. Fig. 7a presents the structure feature of the film after 8 cluster depos-
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Fig. 8. The same as in Fig. 6, but for impact energy of 0.01 eV/ atom.
ited. It was calculated that the coverage of the first epitaxial layer was just 36.7%, and the second and the third layer had the values of 27.3% and 13.3%, respectively. After 22 cluster deposited, five epitaxial layers were formed as shown in Fig. 7b. Fig. 8 is the corresponding diffraction intensity. No oscillations appear in the diffraction intensity, which means that thin film grows in three-dimensional island. It can be understood from the characteristic of single cluster deposition discussed in Section 3.1. It is worth noting some metal thin films will grow in SK mode [21]. In our simulation, just two epitaxial layers were formed at deposition energy of 6 eV/atom. Will thin film growth keep in FM mode or change into SK mode? It will be studied in our further work.
4. Conclusion
Fig. 7. The same as in Fig. 5, but for impact energy of 0.01 eV/ atom.
The influence of the impact energy on the initial fabrication of Al thin films formed by LECBD was investigated by MDS. The collision dynamics of a single cluster deposition was explored in detail. In the case of Al13 cluster impacting on Ni surface, thin film growth was found to be layer-by-layer for low deposition energy (6 eV/atom) and threedimension island for soft-landing (0.01 eV/atom). The studies of energy transfer elucidate that at the earlier stage of the impact event the local heating
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has minor effect to the migration of cluster atoms. The ballistic collision, especially, the collision among moving cluster atoms, which is enhanced by the hard Ni substrate, dominates the lateral spread of cluster atoms on the surface. Upon impacting, Al clusters are first fragmented because of the large cohesive energy ratio between the substrate and the cluster. Then, the front cluster atoms reflect from the hard substrate, and collide with latter arriving cluster atoms. They, thus, obtain transverse kinetic energies and migrate on the surface as single atoms at the early stage of deposition (<1.0 ps). And this transient diffusion is contributive to layer-by-layer growth. While for soft-landing, the incident kinetic energy is too lower to break the clusters. Neighbor clusters simply aggregate each other forming big fcc islands. Thin film will grow in 3D island mode with the local order of fcc structure. These characteristics are quite different from that of Ag cluster on graphite [11]. For the latter, the constituent atoms of the Ag cluster remained highly localized laterally on implantation into the surface. Even at the deposition energy 15 eV, the Ag3 clusters adsorbed on the surface always retained intact. The adsorbed Ag clusters were expected to aggregate to form big islands [11]. It can be understood from the large value of the mass ratio of the projectile atom (Ag) to surface atom (C) and that the covalently bonded graphite substrate is relatively soft. Summarizing those above, it means that the growth mode and structure characteristic formed by LECBD are strongly dependent on the properties of both cluster and substrate materials, as well as the impact energy. Similar effects are to be anticipated in the interaction of other energetic clusters with other metal substrates, and will also control the morphology of the resulting ‘‘cluster assembled films’’.
Acknowledgements This work was partially supported by National Nature Science Foundation of China, Postdoctoral Foundation of China. References [1] P. Jensen, Rev. Mod. Phys. 71 (1999) 1695. [2] Mark Yeadon, Judith C. Yang, Mai Ghaly, Robert S. Averback, J. Murray Gibson, Met. Sci. Eng. B 67 (1999) 76. [3] M. Hou, M.E. Azzaoui, Phys. Rev. B 62 (2000) 5117. [4] E. Bauer, Z. Kristallogr. 110 (1958) 372. [5] W.D. Luedtke, U. Landman, Phys. Rev. B 44 (1991) 5970. [6] I. Yamada, J. Matsuo, Z. Insepov, T. Aoki, T. Seki, N. Toyoda, Nucl. Instrum. Meth. B 164 (2000) 944. [7] Q. Hou, M. Hou, L. Bardotti, B. Prevel, P. Melinon, A. Perez, Phy. Rev. B 62 (2000) 2825. [8] H. Hsieh, R.S. Averback, H. Sellers, C.P. Flynn, Phys. Rev. B 45 (1992) 4417. [9] U. Heiz, F. Vanolli, A. Sanchez, W.D. Schneider, J. Am. Chem. Soc. 120 (1998) 9668. [10] I.M. Goldby, L. Kuipers, B. Von Issendorff, R.E. Palmer, Appl. Phys. Lett. 69 (1996) 2819. [11] S.J. Carroll, S.G. Hall, R.E. Palmer, R. Smith, Phys. Rev. Lett. 81 (1998) 3715. [12] S.J. Carroll, R.E. Palmer, P.A. Mulheran, S. Hobday, R. Smith, Appl. Phys. A 67 (1998) 613. [13] G.J. Ackland, M.W. Finnis, V. Vitek, J. Phys. F 18 (1988) L153. [14] F. Gao, D.J. Bacon, G.J. Ackland, Phil. Mag. A 67 (1993) 275. [15] L. Verlet, Phys. Rev. A 159 (1967) 98. [16] Y.X. Wang, Z.Y. Pan, et al., Nucl. Instrum. Meth. B 180 (2001) 251. [17] G.L. Nyberg, M.T. Kief, W.F. Egelhoff, Phys. Rev. B 48 (1993) 14509. [18] T.J. Moran, I.K. Schuller, R. Ranirez, Phys. Rev. B 49 (8) (1994). [19] W.F. Egelhoff, I. Jacob, Phys. Rev. Lett. 62 (1989) 921. [20] J.W. Evans, D.E. Sanders, P.A. Thiel, Andrew E. Depristo, Phys. Rev. B 41 (8) (1990) 5410. [21] K. Umezawa, S. Nakanishi, T. Yumura, Walter M. Gibson, M. Watanabe, Y. Kido, S. Yamamoto, Y. Aoki, H. Naramoto, Phys. Rev. B 56 (16) (1997) 10585.
Impact energy dependence of Al13 cluster deposition on Ni(0 0 1) surface Y.X. Wang
b
a,b
, Z.Y. Pan a,b,*, Q. Wei a,b, A.J. Du Y. Xu a,b, Y.K. Ho a,b
a,b
, Z. Huang
a,b
,
a Institute of Modern Physics, Fudan University, Shanghai 200433, China Ion beam Laboratory, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
Received 19 June 2001; accepted for publication 20 March 2002
Abstract In this paper, the influence of the impact energy on the initial fabrication of thin films formed by low energy cluster deposition was investigated by molecular dynamics simulation of Al13 clusters depositing on Ni(0 0 1) substrate. In the case of soft-landing, (0.01 eV/atom), clusters are rearranged from Ih symmetry into fcc-like clusters on the surface. Then they aggregate each other, which result in thin film growing in 3D island mode. While, growth will be in layer-by-layer mode at the impact energy of a few electron volt due to the transient lateral spread of cluster atoms induced by dense collision cascade. This effect has been traced to collision cascade inside the cluster, which is enhanced by collision with a hard Ni substrate. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Clusters; Growth; Epitaxy; Aluminum; Nickel
1. Introduction The investigation of the low-energy cluster beam deposition (LECBD) is of great importance in synthesizing nanostructured thin films of novel properties and has attracted considerable attention [1–3]. The growth of a film was known to be a complicated process, which relates to its structure and properties. In general, there are three growth modes for metal particle deposition on metal substrate, that is, the Frank–van deer Mere mode * Corresponding author. Address: Institute of Modern Physics, Fudan University, Shanghai 200433, China. Fax: +86-21-6510-4949/6564-3815. E-mail address: [email protected] (Z.Y. Pan).
(FM, layer-by-layer growth), the Volume-Weber mode (VW, three-dimensional islands growth), and the Stranski–Krastanov (SK, three-dimensional island growth after layer-by-layer growth). Most of the earlier studies were concentrated on the equilibrium thermodynamics and kinetics of growth [4,5]. With these theories, it is difficult to predict the growth mode and dynamics of LECBD, because non-equilibrium dominate the growth process and many parameters, e.g. the specific cluster and substrate material, the impact energy and the temperature, etc., affect the growth mode. In consequence, a diverse behavior of thin film growth can be observed with the change of those parameters mentioned above. So it stimulated a number of theoretical and experimental
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 6 9 1 - 6
Y.X. Wang et al. / Surface Science 512 (2002) 128–134
studies [1–3,6–8]. Heiz group investigated the sizedependent catalytic properties of metal cluster (Ag, Cu and Ni) deposition on the MgO surface [9]. The group of Palmer impacted size-selected cluster ions ðAg n Þ on a covalently bonded substrate (graphite) over the energy range 15–1500 eV. The penetration of cluster into surface versus diffusion and aggregation are studied. It highlights the need to consider the collision geometry associated with cluster–surface interaction [10–12]. However, the mechanisms of thin film growth, especially, the importance of interactions of cluster–cluster and cluster–substrate, which is strongly dependent on the growth conditions, are still required to clarify in detail [5,7,8]. In the present work, Al13 clusters of Ih symmetry were chosen to deposit randomly on the Ni substrate by varying the impact energy, Ein , from 0.01 eV/atom (softlanding) to 10 eV/atom (low energy). In this case, the cohesive energy of Al cluster (2.791 eV/atom) is much lower than that of the substrate (4.435 eV/ atom). The influence of incident energy on growth mode of thin films was investigated by molecular dynamics simulation (MDS). Special attention was put on studying the effect of collision dynamics induced by cluster impact on the growth mode of cluster-assembled thin films.
2. Computational model In the molecular dynamics program, classical equations of motion were set at atomic level and integrated by using the leapfrog form of the Verlet algorithm[15]. The interactions between Al–Al, Al–Ni and Ni–Ni atoms were modeled by Finnis– Sinclair potentials of Ackland et al. [13] which was modified further at close atomic separations by Gao [14]. The Al13 cluster of Ih symmetry was selected as the projectile, which was initially located at a sufficient distance above the Ni surface where the interaction between the cluster and the substrate was negligible. Then it impacted normally onto the surface. The Ni(0 0 1) substrate consisted of 12 atom layers with periodic boundary conditions at the edges (x–y plane). Each layer contained 128 atoms (8 8 unit cell). In order to study the dependence of impact energy on the growth dy-
129
namics, the temperature of the substrate was set at 0 K. To explore the atomic-scale growth mechanism of Al cluster depositions, a simulation model was designed as following. Firstly, a single cluster impingement has been studied in Section 3.1, which was expected to elucidate the dynamics of cluster deposition as well as the lateral migration of supported cluster atoms at the earlier stage. Then, thin film growth of multi-cluster deposition was carried out in Section 3.2, where the Al13 cluster was arranged to vertically drop on the Ni surface one after another. Their impact position and the orientation were chosen randomly.
3. Results and discussion 3.1. Single Al13 cluster interaction with Ni surface In our previous work [16], it was found that Ni substrate is hardly damaged for Al13 cluster impingement with energy lower than 10 eV/atom. Thus, deposition energy of 6 eV/atom was chosen to study the deposition dynamics of Al clusters impacting on Ni(0 0 1) surface. The top and side views of atomic locations of the simulation system at various instants of time during a collision event are shown in Fig. 1. As the Al13 cluster reached the Ni surface at about 0.1 ps, it was deformed severely. At t 0:2 ps, it impinged into fragments mainly being single atoms. Then, these atoms widely outspread as single atoms during the time interval (0.2–0.8 ps). At the end, they resided on the surface as an epilayer. To describe the lateral spread of cluster atoms quantitatively, the radius of gyration ðrg Þ of the cluster atoms [8] was calculated as a function of time, as shown in Fig. 2. It was defined as follows: 1
rg ¼ ðhri2 iÞ2 Here, ri is the transverse distance between the ith cluster atom and the center of mass of the cluster (MCC). It can be seen from Fig. 2 that at t 0:8 ps, rg got to the equilibrium value about 2.93 aNi 0 where aNi 0 is the lattice constant of Ni metal. It is much more than rg ¼ 0:73 aNi 0 at t ¼ 0, i.e. the radius of the original cluster. While, lateral
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Fig. 1. Snapshots of an Al13 cluster of In symmetry impacting on Ni(0 0 1) surface. The incident energy is 6.0 eV/atom. At each instant, the left shows the side view and the right for the top view. The black dots represent Ni atoms, and open circles for Al atoms.
Fig. 2. The radii of gyration of the cluster atoms versus time for an Al13 cluster deposition on (0 0 1) Ni surface with energy of 6.0 eV/atom.
movement of the MCC was only 0.17 aNi 0 . Furthermore, the distribution of transverse spread of the cluster atoms ðri Þ, at t ¼ 0:89 ps is exhibited in Fig. 3, where the statistics were accumulated over eight events and twenty time steps for each event. At the moment, most of cluster atoms have moved more than half of a lattice constant. Some of them
Fig. 3. The distribution of transverse spread of the cluster atoms. The solid line is the distribution at t ¼ 0:89 ps. And the dashed is that of a free Al13 cluster.
were located far from other cluster atoms and the separation between them is beyond the cut off . Among them, the distance of the potential, 4.96 A . We return to Fig. farthest spread ri is up to 16.0 A 2. It can be inferred from the slope of the curve that cluster atoms obtained the maximal transverse velocities at about 0.15 ps. The simulation
Y.X. Wang et al. / Surface Science 512 (2002) 128–134
also showed that within this short time interval, the acceleration process was almost completed and the transverse kinetic energies of cluster atoms relative to the MCC, Epi , raised to the maximum. While, the vertical translational kinetic energy of cluster atoms dropped into the minimum, being close to zero. For those cluster atoms moving far from MCC, the values of Epi were above 0.5 Ein , among them, the highest was close to 0.96 Ein . It illustrates that cluster atoms obtained enough lateral kinetic energy relative to the MCC, and then, spread widely through transient diffusion due to ballistic collision. An epilayer was thus formed. Then we varied the orientation and the impact position of the incident cluster relative to the surface. It was found that the deposition morphology was almost unaffected by the collision geometry, which is different from that on a covalently bonded graphite [11]. Furthermore, the factors, resulting in spreading of cluster atoms so much, were investigated. The local heating caused by cluster impact was first examined. The study of energy transfer showed that the internal kinetic energy of the substrate was increased to 40 eV after deposition [16]. Assuming it was close to the equilibrium state, the corresponding kinetic temperature was around 100 K, since the impinging energy was lower (6 eV/atom). Besides, the time, at which cluster atoms obtained the maximum transverse velocity, was about 0.15 ps. It is much short than the time scale of thermal migration. Therefore, we conclude the impact induced collision cascade takes a leading role, which was related to both cluster–cluster and cluster– surface interaction. Then, we performed two new simulations, which were similar to the previous one to investigate the two effects in detail, respectively. In one simulation, the interaction among the cluster atoms was neglected. The calculation showed the rg was just 1.50 aNi 0 at the end of the event. For the other simulation, the interactions between cluster atoms have been considered, but Al replaced the Ni substrate. In this case, rg was still small, which was calculated to be 1.36 aAl 0 , where aAl is the lattice constant of Al metal. It can 0 be understood qualitatively as follows. The Ni substrate is much harder than Al [8]. Not the same as Al, the Ni substrate does not undergo sub-
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Fig. 4. Collision between two cluster atoms leading to the lateral acceleration of cluster atoms. Front atom A in the Al cluster is assumed to be reflected from the hard Ni surface having the maximum velocity V0 , where V0 is the initial velocity of projectiles. In the left sketch, the collision is inspected from a reference frame moving with atom B. The reflected atom A having the velocity 2V0 collided with atom B. After the collision, both atoms obtained the velocity component parallel to the surface. The right sketch indicates the transformation back to the laboratory frame. The cluster atom gained the maximum lateral velocity, V0 , if it exits at the angle of 45° with the incident direction in the moving frame.
stantial yielding under the Al cluster impact, which makes the substrate response for more elastic stiffness and promotes the cluster atom migration. Furthermore, a rough estimation was performed from the classical two-body collision shown in Fig. 4. Assuming that an Al cluster atom (atom A) reflected normally from the hard substrate with the maximum velocity V0 , where V0 is the initial translational velocity of the projectile, it may collide with another later arriving Al projectile (atom B). Under the condition of one head-on collision between the two moving Al atoms, both can be accelerated in the direction parallel to the surface. This process can be illustrated in a reference frame moving with the cluster velocity. Atom A moving with velocity 2V0 collides with atom B, the two Al atoms must move at right angles to each other after the collision. If one atom exits near the direction of p=4 with the initial beam direction, it will obtain the maximum lateral velocity, V0 , in the laboratory frame, which is close to our simulation result. On the other hand, in the case of monatomic impact, the projectile atom cannot get
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lateral momentum when it collides with a hard substrate. Whereas, the moving Al atom can also obtain the lateral pffiffiffivelocity which maximum value is less than V0 = 3 if it collides with a static Ni atom. Therefore, it is believed the collisions between moving cluster atoms influenced by the hard Ni substrate are important factors in contributing to cluster atoms obtaining high transverse velocity. Consequently, these atoms can widely spread over the Ni surface in the form of single atom motion. Compared with large clusters [8], it seems that the interactions are more efficient for small clusters to spread. As a comparison, a cluster with energy of 0.01 eV/atom (soft-landing) was deposited on the same substrate. It was found that collision cascade did not take place. The Al cluster was supported on the Ni surface as a bulk molecule and rearranged into fcc-like structure due to strong Al–Ni interaction [16]. The transfer between the potential energy and the kinetic energy of this system was completed within 2.0 ps. rg , corresponding to this fcc-like cluster, was calculated to be 0.84 aNi 0 , close to that of a free Al13 cluster, 0.73 aNi The energy 0 released upon formation of the cluster–surface bond is much smaller than the cohesive energy of the Al13 cluster. And then, cluster atoms could not get over the binding among them to spread. 3.2. Simulation of initial fabrication of Al thin film growth We know from Section 3.1 that there is a moderate energy range in which collision cascade results in wild spread for cluster atoms over the surface. Is the behavior of collision dynamics able to influence thin film growth in the moderate energy range? Since the limitation of computational capability, only 22 clusters were calculated to deposit on the surface one after another at the energy of 6 eV/atom. It is known that the mainly dynamics process of deposition has been completed within 3 ps. In addition, the transient process plays a leading role in the metal deposition [17]. So, in our simulation, the deposition was arranged as follows to mimic the experimental conditions. After one Al13 cluster impacted on the surface, the trajectory integration lasted 10 ps to
Fig. 5. The structure feature of Al13 cluster impacting on Ni(0 0 1) surface with energy of 6 eV/atom. (a) The snapshot after 8 clusters impacted on the surface. (b) The snapshot after 22 clusters impacted on the surface.
reach a quasi-equilibrium state. After that, the next cluster began to drop. Fig. 5a is the snapshot after 8 clusters deposited on the Ni surface. It can be seen that all the clusters break away. The atoms in different clusters were intermixed each other. Most of them resided on the surface forming an Al epilayer. For this epilayer, the coverage of Al atoms ðhÞ was 69%. After 22 clusters, in turn, impacted on the surface, two Al epitaxial layers were constructed as shown in Fig. 5b. To monitor its growth mode, we calculated the diffraction intensity [18] as a function of the coverage of cluster atoms, and plotted it in Fig. 6. In Fig. 6, there is a
Fig. 6. Diffraction intensity versus the coverage of cluster atoms for Al13 clusters deposition on Ni(0 0 1) surface with energy of 6 eV/atom.
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peak near h 1:0. This indicates the initial thin film growth is layer-by-layer like during Al cluster deposition. It can be understood from Section 3.1. The cluster was breaking into pieces and the projectile atoms obtained transverse momentum due to the impingement. Then the cluster atoms experienced transient diffusion to move far from the impact point over the surface. The supported clusters were coalesced each other. Hence, the layerby-layer growth of deposited clusters was thus formed. This mechanism of transient mobility for film growth at low temperature is different from that for film growth in atomic deposition [19,20]. In the case of atom deposition, the transient diffusion stems from the need to dissipate the kinetic energy of single atom and the potential energy released upon formation of the atom–surface bond. It might be interesting to study how does the growth mode of cluster-assembled films depend on the impact energy? Therefore, the simulation was repeated again, but the deposition energy per atom was 0.01 eV. As expected, no fragments generated. Each cluster just rearranged into fcc structure. Additionally, we did not find the collective motions of deposition clusters. The neighbor clusters simply aggregate each other into big fcc island [7]. Subsequently, fcc islands grew up through the aggregation. Fig. 7a presents the structure feature of the film after 8 cluster depos-
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Fig. 8. The same as in Fig. 6, but for impact energy of 0.01 eV/ atom.
ited. It was calculated that the coverage of the first epitaxial layer was just 36.7%, and the second and the third layer had the values of 27.3% and 13.3%, respectively. After 22 cluster deposited, five epitaxial layers were formed as shown in Fig. 7b. Fig. 8 is the corresponding diffraction intensity. No oscillations appear in the diffraction intensity, which means that thin film grows in three-dimensional island. It can be understood from the characteristic of single cluster deposition discussed in Section 3.1. It is worth noting some metal thin films will grow in SK mode [21]. In our simulation, just two epitaxial layers were formed at deposition energy of 6 eV/atom. Will thin film growth keep in FM mode or change into SK mode? It will be studied in our further work.
4. Conclusion
Fig. 7. The same as in Fig. 5, but for impact energy of 0.01 eV/ atom.
The influence of the impact energy on the initial fabrication of Al thin films formed by LECBD was investigated by MDS. The collision dynamics of a single cluster deposition was explored in detail. In the case of Al13 cluster impacting on Ni surface, thin film growth was found to be layer-by-layer for low deposition energy (6 eV/atom) and threedimension island for soft-landing (0.01 eV/atom). The studies of energy transfer elucidate that at the earlier stage of the impact event the local heating
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has minor effect to the migration of cluster atoms. The ballistic collision, especially, the collision among moving cluster atoms, which is enhanced by the hard Ni substrate, dominates the lateral spread of cluster atoms on the surface. Upon impacting, Al clusters are first fragmented because of the large cohesive energy ratio between the substrate and the cluster. Then, the front cluster atoms reflect from the hard substrate, and collide with latter arriving cluster atoms. They, thus, obtain transverse kinetic energies and migrate on the surface as single atoms at the early stage of deposition (<1.0 ps). And this transient diffusion is contributive to layer-by-layer growth. While for soft-landing, the incident kinetic energy is too lower to break the clusters. Neighbor clusters simply aggregate each other forming big fcc islands. Thin film will grow in 3D island mode with the local order of fcc structure. These characteristics are quite different from that of Ag cluster on graphite [11]. For the latter, the constituent atoms of the Ag cluster remained highly localized laterally on implantation into the surface. Even at the deposition energy 15 eV, the Ag3 clusters adsorbed on the surface always retained intact. The adsorbed Ag clusters were expected to aggregate to form big islands [11]. It can be understood from the large value of the mass ratio of the projectile atom (Ag) to surface atom (C) and that the covalently bonded graphite substrate is relatively soft. Summarizing those above, it means that the growth mode and structure characteristic formed by LECBD are strongly dependent on the properties of both cluster and substrate materials, as well as the impact energy. Similar effects are to be anticipated in the interaction of other energetic clusters with other metal substrates, and will also control the morphology of the resulting ‘‘cluster assembled films’’.
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